The latest version of Luma does not support Arm9loaderhax. Go to Luma3DS's github and download version 7.0.5 and drag the arm9loaderhax.bin to the root of your sd card and follow https://3ds.guide/a9lh-to-b9s when your done to update correctly.

If the boot.firm from Luma3DS 7.1 worked, then you definitely have B9S already installed. However, because it's an older version of B9S, it's not compatible with Luma3DS 8.0. So, you need to follow the above guide to update B9S. Then Luma3DS 8.0 will work.

If the boot.firm from Luma3DS 7.1 worked, then you definitely have B9S already installed. However, because it's an older version of B9S, it's not compatible with Luma3DS 8.0. So, you need to follow the above guide to update B9S. Then Luma3DS 8.0 will work.